8. THE MASS DENSITY IN BLACK HOLES

Recent dynamical evidence indicates that supermassive black holes reside
at the center of most nearby galaxies. The available data (about 30 objects)
show a strong correlation (but with a large scatter) between bulge and black
hole mass
[38],
with Mbh = 0.006 Mbulge as a
best-fit. The total mass density in spheroids today is
bulge =
0.0036+0.0024-0.0017
[16],
implying a mean mass density of dead quasars

(6)

Since the observed energy density from all quasars is equal to the
emitted energy divided by the average quasar redshift
[49], the
total contribution to the EBL from accretion onto black holes is

(7)

where 0.1
is the efficiency for transforming accreted rest-mass
energy into radiation (in units of 10%). Quasars at
z 2 could then
make a significant contribution to the brightness of the
night sky if dust-obscured accretion onto supermassive black holes is an
efficient process
[21],
[14]
(4).